Gas fireplaces create flames by burning natural gas rather than wood. Ceramic logs and embers sitting in a metal grate give it the appearance of a wood-burning fire, but they are just for show. Like a standard fireplace, a gas unit produces both heat and waste gases. Your home is shielded from both by a glass barrier. The deflected heat and gases are vented out of your home through the fireplace chimney.
To be up to code, all fireplaces must have a damper. A functional damper opens and closes easily just by using the handle or lever; any damper that requires extra work to move is considered nonoperational and should be repaired. The damper in a gas fireplace should remain open all the time. The glass between the firebox and your home prevents cold or heat from coming down the chimney and by leaving the damper open at all times, you eliminate the risk of having gas build up or backdraft into your home.
The exception to the open damper rule is in a vent-free gas insert. These units are designed to be clean-burning and to produces low levels of waste gas, so they do not need to be vented through the damper and out the chimney. They produce more heat than vented gas fireplaces.
When converting your wood-burning fireplace to natural gas, have the work done by an experienced professional. That ensures both the installation follows local building codes and is safe. Before using your gas fireplace for the first time each year, check to see that the damper is open and that the portion of the chimney you can see is clean and soot- or debris-free. Have the chimney cleaned on a regular basis so no fallen bird nests or branches interfere with its function.